Mother crashed shortly after midnight last night. I didn’t find out about it until this morning.
This has been the 2nd crash in the last two months, since last time. A bit too frequent for comfort, so instead of asking the KDDI staff to reset mother, I went to the data centre to conduct a post-mortem.
The omniscient Google points to memory exhaustion and suggested increasing the MAX_KMAPENT parameter in the kernel. I think this has something to do with me having been running the Squid (which I believe is still leaking memory slowly) proxy on mother for my own use. I am going to patch up and recompile the kernel and we’ll see if this solves the problem.
BTW it took an hour for mother to rebuild the software RAID and come back up after the reboot.